An ion implantation apparatus used in manufacturing semiconductors implant impurities in targets such as silicon or gallium arsenide substrates. Such ion implantation apparatuses extract an ion beam from an ion source, accelerate the ion beam to a desired implantation energy, and direct the ion beam at a target to implant the ions in the target. The concentration of the impurity being introduced into the target can be established by measuring the current of the ion beam using a Faraday cup and integrating the current over time to determine the dose. For current integrated circuit manufacturing processes, high energy implant processes that can freely control impurity profile in the interior of silicon substrates are increasingly important. To achieve high energy implants, implantation apparatuses commonly use tandem acceleration principles for accelerating ions to high energy and implanting them in silicon substrates. Tandem acceleration principles are well known and are described in U.S. Pat. No. 3,353,107, which is hereby incorporated herein its entirety. Tandem acceleration techniques produce a negative ion beam by combining a positive ion source and a charge exchange cell, or by using a sputter type negative ion source. The negative ion beam is directed toward an accelerator terminal that is at high positive voltage and accelerated to the terminal voltage. Electrons are then stripped from this accelerated negative ion beam by passing the beam through a gas or thin foil, to convert the beam into a positive ion beam. The positive ion beam is accelerated again to ground potential from the accelerator terminal maintained at high positive potential and acquires its final energy.
An example of an actual apparatus that uses this tandem principle is a Genus Inc. Model G1500 high energy ion implantation apparatus. FIG. 1 shows the model G1500 modified by omitting a pre-acceleration tube. U.S. Pat. No. 4,980,556 further describes such ion implantation devices and is hereby incorporated by reference herein in its entirety. In this apparatus, a hot-cathode PIG (Penning Ion Gauge) ion source 1 produces positive ions that are extracted as a beam by impressing a high positive voltage on ion source 1. The extracted positive ion beam collides with magnesium vapor when passing through a charge exchange cell 2 which is adjacent the extraction electrode system. In the collisions, some of the positive ions in the positive ion beam pick up two electrons from the magnesium vapor and are converted to negative ions.
After passing through the charge exchange cell 2, a 90-degree analyzing magnet 3 analyzes the beam according to the charge-to-mass ratio of the ions so that only the desired negative ions are injected into a tandem accelerator 5. A quadrapole magnetic lens (Q-lens) 4 at the entrance aperture part of the low-energy acceleration tube 6 of the tandem accelerator 5 receives the mass-analyzed negative ion beam and focuses the beam to create a beam waist at the center of a stripper canal 7 which is in tandem accelerator 5. At the same time, a high positive voltage accelerates the negative ion beam towards stripper canal 7.
When this accelerated negative ion beam passes through the stripper canal 7, ions lose orbital electrons in collisions with nitrogen gas in stripper canal 7 and are converted again into positive ions. At this time, the energy of the collisions determines the distribution of charge states of the ions in the beam. In particular, higher energy collisions produce more multi-charged ions. The positive ion beam thus obtained is directed towards ground potential from the tandem accelerator terminal, and further accelerates while passing through a high-energy acceleration tube 8.
The beam thus having its final energy receives further focusing by a quadrapole lens 9. An analyzing magnet 10 selects ions having the desired charge state and directs the selected ions into a process chamber 11 containing a target (for example, wafer or substrate).
The ion implantation apparatus of FIG. 1 generally includes two Faraday cups (or cages). The first Faraday cup is between analyzing magnet 3 and Q-lens 4 and measures the beam current for set up the beam. The second Faraday cup is within the process chamber 11 and measures the implant dose by measuring the beam current. Hereinafter, the first Faraday cup is called the "injector Faraday cup" and the second Faraday cup is called "disk Faraday cup".
FIG. 2 illustrates a beam current 21 striking a disk 23 at a radius 24. Disk 23 holds a wafer 25 and rotates to scan beam 21 across wafer 25. Disk also moves up and down to change the radius at which beam 21 crosses wafer 25. A disk Faraday cup 22, which is behind disk 23, samples beam current 21 each time disk 23 rotates so that beam current 21 passes through a gap in disk 23. At the end of a scan across wafer 25, beam 21 is at radius 27 and no longer impacting upon wafer 25, and injector Faraday cup 26, which is upstream of spinning disk 23, moves in position to measure the upstream current.
In the above-described ion implantation apparatus, the injector and disk Faraday cups have bias rings that are electrically connected in series. The bias rings have a bias voltage that suppresses capture of electrons by the respective Faraday cups. Capture of electrons can lead to errors in measurement of the current and the implantation dose, resulting in failure of an integrated circuit being manufactured. However, the known ion implantation apparatus is unable to monitor whether both bias rings are appropriately biased.